Sheldon Hambrick , Jun 20, 2006; 03:50 p.m.
I'd like to try my Canon 2X extender on a friend's EF 300/4.0 L (non-ISM).
Since I'm not using a series-1 body, I'd like to play around with retaining AF
at f8. Looking at the rear of the converter, counting clockwise, which
contacts do I need to tape over?
Thanks in advance.
Jamie Nicholson , Jun 20, 2006; 04:19 p.m.
Looking at the lens side of the extender with the pins at the top, tape off the three left pins.
Sheldon Hambrick , Jun 20, 2006; 04:22 p.m.
Scott Matchunis , Jun 20, 2006; 04:34 p.m.
Why does taping over the pins allow autofocus to work on a teleconverter that Canon recommends not using on a specific lens? Asked another way, why does Canon not want consumers to use certain teleconverter/lens combinations? If my Canon EF100-400 IS lens is not supposed to be used with a 2X teleconverter (according to Canon) why would I want to tape the pins and try it?
Alistair Windsor , Jun 20, 2006; 04:53 p.m.
The pins inform the camera that the convertor is there so the camera can adjust the aperture. With a 2x convertor the 300/4 becomes an 800/8. The consumer bodies only do autofocus to f5.6 and hence autofocus is deactivated. Taping the pins means the camera doesn't know the convertor is present. It thinks the maximum aperture is f4 and hence attempts to autofocus. Unless there is a lot of light and a subject with a lot contrast the AF will fail (since the maximum aperture is really f8). If you add a teleconvertor to a lens and the combination is supposed to AF then AF is slowed to improve AF accuracy. Taping pins can also eliminate this slowing. Both the slowing and the maximum aperture restriction are put there by Canon to ensure reasonably reliable AF. Try taping pins and you can expect hunting and occasional misfocusing. My 70-200/4 + 1.4x TC and 300/4 + 1.4x TC are both within specs for AF but in low light AF struggles. Having said that optically both combinations are superb.
Metering is largely unaffected by taping the pins (there is a slight effect since the camera meters wide open and uses its knowledge of the maximum aperture to adjust for certainly nonlinearities in the transmission of the screen but the effect is very minor with slow lenses).
Bjorn Tennoe , Jun 21, 2006; 12:24 a.m.
I have tried tape on the combination 300 f4 (non-IS) and EF 2x (old version). It did not work, the AF was only hunting without finding focus.
Steve Dunn , Jun 21, 2006; 02:56 p.m.
Why does taping over the pins allow autofocus to work on a teleconverter that Canon recommends not using on a specific lens?
Canon doesn't recommend against this combination. They simply state that if you use a body+TC combination that's slower than the minimum aperture required by the body's AF system, you don't get AF.
I have tried tape on the combination 300 f4 (non-IS) and EF 2x (old version). It did not work, the AF was only hunting without finding focus.
Some models of bodies have AF systems which may work a bit farther beyond their official limit than others. As well, it depends on how well the target was illuminated and if it had a good, high-contrast feature for the AF system to lock onto. In some very brief and unscientific testing, I found that my 20D's AF system had somewhat more ability to work at f/8 than my Elan 7E did. But neither one was great, which is to be expected, since both are designed to work only to f/5.6.
John Cant , Aug 27, 2011; 08:35 p.m.
I have a 40D, 300mm f4 lens and have tried a 2x extender which I just bought. It works fine manual focus, but when I follow the instructions above about taping the three left contacts, the lens hunts and never settles on being in focus. This was in bright sunlight. (I used electrician's tape.) Any further suggestions?
Thank you.
